Abstract: This chapter examines controversies surrounding the U.S. Constitution’s relationship to international law, with particular emphasis on the separation of powers, federalism, the supremacy clause, and individual rights. Before discussing tensions between “original” understandings of international law and the U.S. Constitution, the chapter provides an overview of the founding of the Constitution and its focus on international affairs and foreign relations. It then considers what the Constitution says about treaty law and other international agreements, along with U.S. law’s jurisdiction and supremacy over customary international law. Finally, it explores how international law is interpreted by U.S. law, and how the overlap between international law and constitutional law has increased.